In an art to clean contaminated soil, an easy and inexpensive method is desired in view of industrially implementing it. As a conventional art, one of methods to clean soil contaminated by heavy metals, oil, and the like is a soil washing method. This method utilizes a characteristic that in contaminated soil, a larger amount of the contaminant adheres to soil particles having a smaller particle size, and it is a method to turn the contaminated soil into slurry by using a large amount of water and perform wet classification to remove fine products. This method, however, has a problem that loads of a disposal step of contaminated water produced when the soil is turned into the slurry and a dehydration step of treatment residues are large. Another problem is that, when the content of the contaminants in the contaminated soil is high, it is not possible to remove the contaminants except large gravel to produce clean soil.
Therefore, in recent years, there has been proposed a method in which a small amount of quicklime is mixed in contaminated soil, a wet granular lump is contained in the soil that is to be treated, and a classifying means and a means for crushing the granular lump are combined to remove a clay fraction from the soil (Patent Document 1). Further, as a means for obtaining clean soil from heavily contaminated soil, there have been proposed a method to feed contaminated soil to a magnetic separator to collect contaminants in the soil as magnetically attracted matter and a method to separate and remove contaminants by combining an ore flotation method with the soil washing method (Patent Documents 2, 3).    [Patent Document 1] Japanese Patent Application Laid-open No. 2008-289963    [Patent Document 2] Japanese Patent Application Laid-open No. 10-71387    [Patent Document 3] Japanese Patent Application Laid-open No. 2003-103248